In a paper making process of manufacturing paper continuously, it is important to control the water content. This necessitates a water content measuring apparatus for measuring, online, the water content of paper that is being moved in a paper making line. Among several types of water content measuring apparatus that have been put into practice, an infrared water content measuring apparatus using near infrared light is used widely as an online water content measuring apparatus.
In infrared water content measuring apparatus, radiation having such a wavelength as to be absorbed by water and not to be absorbed by cellulose which is the main component of paper and radiation having such a wavelength as to be absorbed by cellulose and not to be absorbed by water are caused to pass through a paper as a measurement target. A water content of the paper is calculated on the basis of an intensity ratio between radiation beams having the respective wavelengths measured by a radiation receiver. Reference radiation having such a wavelength as to be absorbed by neither water nor cellulose may also be used to eliminate influences of radiation scattering by the paper, substances mixed in the paper, the basis weight, the ash content, the lignin content, colorants used, a coating, and other factors.
FIG. 6 is a block diagram showing the configuration of a related-art infrared water content measuring apparatus 40. As shown in FIG. 6, in the related-art infrared water content measuring apparatus 40, a halogen lamp 410 having a broad, continuous spectrum is generally used as a radiation source. Radiation emitted from the halogen lamp 410 is guided, via a projection lens 411, to band-pass filters 421 which are mounted on a filter wheel 420.
In the example of FIG. 6, three kinds of band-pass filters, that is, a band-pass filter 421a which transmits reference radiation having such a wavelength λa as to be absorbed by neither water nor cellulose, a band-pass filter 421b which transmits radiation having such a wavelength λb as to absorbed by water and not to absorbed by cellulose, and a band-pass filter 421c which transmits radiation having such a wavelength λc as to absorbed by cellulose and not to absorbed by water, are mounted on the filter wheel 420. However, there are other cases in which four or more band-pass filter 421 are used to produce plural reference radiation beams having different wavelengths.
Radiation beams having the respective wavelengths are output sequentially in accordance with rotation of the filter wheel 420. It is assumed that, as shown in a broken-like rectangle A, the radiation beam having the wavelength λa is output at time point t1, the radiation beam having the wavelength λb is output at time point t2 and the radiation beam having the wavelength λc, is output at time point t3. To prevent reduction of measurement accuracy due to inclusion of components having other wavelengths, each band-pass filter 421 is required to have a specification in which a half-value width of a spectrum of the output radiation beams is an extremely narrow.
Radiation that has passed through the band-pass filter 421 is applied onto paper 700 as a measurement target. To increase the sensitivity, the infrared water content measuring apparatus 40 is configured in such a manner that the paper 700 is interposed between a top reflector 431 having an entrance hole and a bottom reflector 432 having an exit hole so that radiation passes through, many times, the paper 700 which is moving in a paper making line.
Radiation that is output through the exit hole of the bottom reflector 432 is detected by a radiation receiver 440 such as a PbS cell and a resulting detection signal is amplified by an amplifier 441. As shown in a broken-line rectangle B, amplified detection signals are obtained in time-series at time points corresponding to time points of output of radiation beams from the respective band-pass filters 421. More specifically, a signal Va which is obtained at time point t1 corresponds to intensity of the radiation beam having the wavelength λa, a signal Vb which is obtained at time point t2 corresponds to intensity of the radiation beam having the wavelength λb, and a signal Vc which is obtained at time point t3 corresponds to intensity of the radiation beam having the wavelength λc.
The amplified detection signals are input to a processing unit 450, where an index value calculator 451 calculates an index value on the basis of an intensity ratio between the detection signals corresponding to the respective wavelengths. Then, a water content calculator 452 converts the index value into a water content by referring to a calibration curve 453 which was prepared in advance, and outputs information of the water content. The calibration curve 453 is data which correlates correct water contents measured by an electronic balance or the like with index values measured by the infrared water content measuring apparatus 40.